Point out where command docs should be edited (#1042)

make_docs.nu

@@ -255,14 +255,15 @@ def generate-command [commands_group command_name] {
     let doc_path = (['.', 'commands', 'docs', $'($safe_name).md'] | path join)
 
     let frontmatter = (command-frontmatter $commands_group $command_name)
+    let note = "<!-- This file is automatically generated. Please edit the command in https://github.com/nushell/nushell instead. -->"
     let doc = (
         $commands_group
         | get $command_name
         | each { |command| command-doc $command }
         | str join
     )
 
-    [$frontmatter $doc] | str join "\n" | save --raw --force $doc_path
+    [$frontmatter $note $doc] | str join "\n" | save --raw --force $doc_path
     $doc_path
 }